In this contribution we describe a comprehensive investigation of the proton transfer (PT) mechanisms involved inside aqueous, solvent-separated encounter complexes using ab-initio molecular dynamics (AIMD). This model framework employed can be viewed as a ground-state analog of the excited state proton transfer reactions that have been actively investigated using ultrafast spectroscopy by many research groups. The AIMD complements these experiments extremely well, giving access to details of the underlying mechanisms that are currently impossible to obtain from the experimental data alone.

Specifically, we study direct charge transfer in a donor-bridge-acceptor system that appears to follow two pathways: a “concerted” one in which spectroscopic signatures of acid dissociation are followed by those characteristic to base protonation in significantly less than 1 ps and a “sequential” pathway, in which the two aforementioned events are separated by several picoseconds. We are able to formulate a global reaction mechanism, compute elementary rate constants for these processes and show that the fast and slow pathways are a consequence of the participation of the ubiquitous Eigen and Zundel forms of the hydronium ion, respectively. Most importantly however, the stability and reactivity of the Zundel and Eigen forms has been significantly altered by the local H-bond network of, and electrical charge distribution inside the encounter pair. In the present acid-base reaction the Zundel form along the pathway significantly slows down the overall process, and appears to act as an effective ‘dead end’ for the overall charge transfer, which is in sharp contrast with the “transition state” role that the Zundel ion plays in the case of proton transfer in bulk water. Such reversals of the role played by Eigen and Zundel forms have been previously reported for example in the context of proton transfer along one-dimensional water chains confined in carbon nanotubes. However, to the best of our knowledge our investigation is the first to report on the relative stability and mechanistic role played by Zundel and Eigen forms in encounter complexes, which are known to play a key role in all acid-base reactions in aqueous environments.

The results presented represent a significant step forward in our molecular level understanding of the diverse processes involved in proton transfer within water separated encounter complexes.